1. Field of the Invention
The present invention relates to a gas turbine combustor for mixing fuel into combustion air introduced from a compressor, burning an air-fuel mixture, and supplying produced combustion gas to a gas turbine. More particularly, the present invention relates to a gas turbine combustor capable of burning either one or both of liquid fuel and gaseous fuel, and to a fuel supply method for the gas turbine combustor.
2. Description of the Related Art
Recently, a demand for higher output and higher efficiency of gas turbine plants has increased, and the temperature of combustion gas tends to rise year by year. Higher temperatures of the combustion gas increase the concentration of nitrogen oxides (hereinafter expressed by NOx) contained in gas turbine exhaust gas correspondingly. In the field of gas turbine combustors, therefore, how to reduce NOx emissions has become an important problem from the viewpoint of protecting the global environment.
With such background in mind, a gas turbine combustor has hitherto been proposed which employs a premixed combustion method capable of avoiding local generation of high-temperature combustion gas and reducing NOx emissions by jetting out fuel from a nozzle into the high-temperature combustion gas and burning an air-fuel mixture after uniformly mixing the fuel and the combustion air in advance.
One example of the gas turbine combustor employing the premixed combustion method comprises a pilot fuel nozzle for producing combustion gas by diffusion combustion, a plurality of main fuel nozzles disposed around the pilot fuel nozzle, a premixing duct formed with a diameter gradually reducing toward the downstream side in the flow direction and mixing fuel jetted out from the main fuel nozzles into introduced combustion air, and a combustion chamber in which premixed gas introduced from the premixing duct is burnt with the diffusion combustion gas acting as an ignition source (see, e.g., Patent Reference 1; JP,A 9-264536). With such a gas turbine combustor, because the premixing duct has a length sufficient to uniformly mix the combustion air and the fuel, homogeneous premixed gas can be produced and hence NOx emissions can be reduced.
The above-described related art, however, has problems given below.
According to the known gas turbine combustor described above, because the premixing duct has a length sufficient to uniformly mix the combustion air and the fuel, the premixed gas is filled in the premixing duct, thus leading to a risk of spontaneous ignition of the premixed gas in the premixing duct or flushing-back of a flame into the premixing duct from the combustion chamber. Also, since dust or the like is often mingled in the combustion air introduced to the combustor during a process in which the combustion air is produced with compression by a compressor and then flows down through channels, the mingled dust or the like may be contained in the combustion air introduced to the premixing duct. If the dust or the like is a combustible material, it may be heated and ignited by the combustion air at high temperatures. In such an event, there is a risk that a flame may remain in an upstream area of the premixing duct where the gas flow speed is relatively low, due to the above-mentioned structure that the premixing duct is formed with a diameter gradually reducing toward the downstream side. The occurrence of that event may bring about overheating of the premixing duct to cause a deformation or breakage thereof, and hence may invite a risk of damage of the gas turbine in its entirety.
With the view of overcoming the above-described problems in the related art, it is an object of the present invention to provide a gas turbine combustor and a fuel supply method for the gas turbine combustor, which can prevent flushing-back of a flame while reducing NOx emissions.